R—Fe—B based permanent magnets have high magnetic characteristics and thus are used in various industrial products, including motors for electric power steerings, engine motors of hybrid electric vehicles or electric vehicles, motors for air-conditioners, magnetic head actuators for hard disk drives, and the like. However, they have the property that their coercive force decreases at high temperatures. Accordingly, R—Fe—B based permanent magnets to be incorporated into motors for use in vehicles, etc., are required to have particularly high coercive force so that predetermined coercive force can be maintained even after exposure to high temperatures in severe use environments. Under such circumstances, attempts have been made to improve the coercive force of R—Fe—B based permanent magnets. As a method therefor, a method in which a heavy rare earth element, such as Dy or Tb, is added to a raw material alloy for R—Fe—B based permanent magnets is known. This method is advantageous as a method for improving the coercive force of R—Fe—B based permanent magnets, but in some cases, the heavy rare earth element is added to a content near 10 mass % in the magnet. However, heavy rare earth elements are scarce resources, and our country is dependent on imports from China. Therefore, there is urgent need of reducing use of heavy rare earth elements as much as possible. Thus, as a method for achieving the efficient improvement of the coercive force of an R—Fe—B based permanent magnet with reduced use of heavy rare earth elements, a method in which a heavy rare earth element is diffused from the surface into the inside of an R—Fe—B based permanent magnet has been attracting attention. For example, Patent Document 1 proposes a method in which a magnet and a diffusion source made of an alloy of a heavy rare earth element and iron (e.g., an alloy piece made of DyFe2, DyFe3, TbFe2, TbFe3, etc.) for diffusing a heavy rare earth element into the magnet are heated while being moved continuously or intermittently in a treatment chamber.
The method described in Patent Document 1 is advantageous as a method in which a heavy rare earth element can be effectively diffused, with reduced use, into an R—Fe—B based permanent magnet to improve the coercive force. However, with respect to the diffusion source made of an alloy of a heavy rare earth element and iron used in this method, the present inventors have found that after repeated use, the heavy rare earth element content of the diffusion source decreases. According to the study by the present inventors, this is attributable to that when an R—Fe—B based permanent magnet and the diffusion source are heated while being moved continuously or intermittently in a treatment chamber, the surface of the diffusion source is fractured, and the resulting fragments of the diffusion source adhere to the surface of the magnet, while the surface of the magnet is fractured, and the resulting fragments of the magnet adhere to the surface of the diffusion source, for example. When the heavy rare earth element content of the diffusion source decreases, the efficiency of the diffusion of the heavy rare earth element into the R—Fe—B based permanent magnet also decreases, and thus the use of the diffusion source is stopped at a certain point. The problem here is how to treat the used diffusion source. Even if it cannot be used as a diffusion source any longer, a heavy rare earth element, which is a scarce resource, is contained therein. Therefore, rather than discarding the used diffusion source, how to recover and recycle the heavy rare earth element contained therein is an important technical challenge for the future.
Several methods have been proposed as methods for recovering a rare earth element from a workpiece containing at least a rare earth element and an iron group element. For example, Patent Document 2 proposes a method in which a workpiece is heated in an oxidizing atmosphere to convert the contained metallic elements into oxides, followed by mixing with water to form a slurry; hydrochloric acid is added with heating to dissolve a rare earth element in a solution; an alkali (sodium hydroxide, ammonia, potassium hydroxide, etc.) is added to the obtained solution with heating, thereby precipitating an iron group element leached into the solution together with the rare earth element; then the solution is separated from undissolved substances and the precipitate; and oxalic acid, for example, is added to the solution as a precipitant to recover the rare earth element in the form of an oxalate. This method is noteworthy as a method that allows a rare earth element to be effectively separated from an iron group element and recovered. However, the method has a problem in that because an acid and an alkali are used in part of the process, it is not easy to control the process, and also the recovery cost is high. Therefore, it must be said that in some aspects, the method described in Patent Document 2 is difficult to put into practical use as a recycling system that is required to be low-cost and simple.
In addition, as a method for not oxidizing an iron group element contained in a workpiece but oxidizing only a rare earth element contained in the workpiece to thereby separate the two, Patent Document 3 proposes a method in which a workpiece is heated in a carbon crucible. Unlike the method described in Patent Document 2, this method does not require an acid or an alkali. In addition, when a workpiece is heated in a carbon crucible, theoretically, the atmosphere in the crucible is autonomously controlled to an oxygen partial pressure at which iron group elements are not oxidized and only rare earth elements are oxidized. Accordingly, this method is likely to be more advantageous than the method described in Patent Document 2 in that the process is simpler. However, when it comes to the question whether the atmosphere in a carbon crucible is autonomously controlled to a predetermined oxygen partial pressure by merely heating a workpiece in a the crucible, whereby rare earth elements can be separated from iron group elements, the reality is not necessarily so. Patent Document 3 states that the oxygen content of the atmosphere in a crucible is preferably 1 ppm to 1%, but essentially no external operation is required to control the atmosphere. However, according to the study by the present inventors, at least in the case where the oxygen content is less than 1 ppm, rare earth elements cannot be separated from iron group elements. Therefore, even if it is theoretically possible that when a workpiece is heated in a carbon crucible, the atmosphere in the crucible is autonomously controlled to an oxygen partial pressure at which iron group elements are not oxidized and only rare earth elements are oxidized, in reality, the inside of the crucible has to be artificially controlled to an atmosphere having an oxygen content of 1 ppm or more. Such control can be achieved by introducing an inert gas having an oxygen content of 1 ppm or more into the crucible, as also described in Patent Document 3. However, in the case of argon gas, which is widely used as an industrial inert gas, its oxygen content is usually 0.5 ppm or less. Therefore, for introducing argon gas having an oxygen content of 1 ppm or more into a crucible, the widely used argon gas cannot be directly used, and it is necessary to especially increase the oxygen content before use. Consequently, although the process of the method described in Patent Document 3 looks simple, actually it is not. It must be said that like the method described in Patent Document 2, in some aspects, the method described in Patent Document 3 is difficult to put into practical use as a recycling system that is required to be low-cost and simple.